To form a solid, molecules need to pack together nicely, while in a liquid there is less order and the molecules flow around each other. Fat molecules are mostly made up of long, straight hydrocarbon chains. Because they are straight they pack neatly with their neighbours (think of the way uncooked spaghetti packs together in a jar).

Oils generally have chains which are kinked, this stops them interacting so tidily and so they stay liquid (imagine the storage problems you’d have if there was a bend in the middle of every piece of spaghetti). By definition, fatty molecules that form liquids are called oils and those that form solids are called fats.

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